The present invention relates to the characterization of solid samples, e.g., thin films. More particularly, the present invention pertains to the use of nondestructive techniques to characterize such samples.
Characterization or analysis of samples (e.g., thickness of a thin film, elemental and/or chemical species concentration in a thin film formed on a substrate, etc.) is necessary in the manufacture of many different types of devices (e.g., electronic and optical electronic devices). For example, it may be necessary to determine the composition of thin dielectric films (e.g., gate oxide films, tantalum nitride films, etc.) formed in known semiconductor integrated circuit devices, such as processing devices and memory devices. Increases in the density of such devices on an integrated circuit chip and reduction in device dimensions require the advancement of production processes and characterization technologies related to the materials used to fabricate such devices.
For example, recent developments in the fabrication of semiconductor devices may employ shallow implant and/or other ultra-thin structures. In one particular example, gate oxide layers have become very thin films, typically in the range of about 1 to 10 nanometers in thickness. Such thin films are difficult to characterize. Such structures will require characterization techniques that have improved sensitivity over conventional characterization techniques.
Further, such techniques may also require the characterization to be performed with ample speed. For example, when such characterization techniques are used to monitor manufacturing tools or processes, e.g., metrology for wafer level manufacture of various films on substrates, the characterization must be done at suitable process speeds. Further, in monitoring of the manufacturing process, preferably, it is desired that such characterization be performed in a nondestructive manner, e.g., using a noninvasive process.
In addition to performing such process monitoring characterization of thin films noninvasively and with ample speed, one must be able to carry out such characterization techniques with precision on a consistent basis. In other words, measurements made or parameters determined using the characterization process must be repeatable with a suitable measurement precision (e.g., relative standard deviation, RSD). In such a manner, a process excursion, e.g., a process not performing as it is intended which typically results in product being produced that is not acceptable, can be detected.
Various techniques have been used for characterization of materials, e.g., to provide thickness measurements and/or to determine the concentration of trace and/or major components in such materials. For example, several of such methods include ellipsometry methods, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), secondary ion mass spectrometry (SIMS), x-ray photoelectron spectrometry (XPS) (also known as electron spectroscopy for chemical analysis (ESCA)), Auger electron spectrometry (AES), and other electron beam methods.
Many of such techniques are sensitive to the near-surface region of a material. Further, many of these techniques also permit a measurement of material properties as a function of depth beneath the surface through depth profiling. In typical depth profiling, for example, continuous or periodic ion beam sputtering removes material from the surface of a sample to expose progressively deeper material at one or more various depths of the sample for further measurement and/or analysis. Generally known sputter rates may be used to determine the depth at which the surface measurements are completed. As such, a characterization of the sample as a function of depth beneath the surface can be attained. However, in process monitoring, such depth profiling techniques are in many circumstances inadequate. For example, depth profiling is an invasive and destructive process, and further, such processes generally take a relatively long period of time to complete, e.g., as compared to just surface measurements.
Further, even if such techniques are not used in a destructive depth profiling fashion, they are typically inadequate in many other respects with regard to characterization of various types of samples. This is particularly the case with respect to characterization of thin films, e.g., thin gate oxide films.
Optically based ellipsometry methods have been the standard method for monitoring SiO2 gate films. However, the change in gate materials, from for example, SiO2 to SiOxNy, has made a major impact on the usefulness of optical measurement tools. The index of refraction changes when nitrogen is added to the film. Since nitrogen content varies with depth, the index of refraction of the films is a variable making it difficult to use standard optical methods to monitor SiON films. In addition, the trend to thinner films (e.g., 18 angstroms, 12 angstroms, 8 angstroms . . . ) is challenging the fundamental limits of optical methods. The combination of these two effects has reduced measurement precision for thickness using such techniques rendering them ineffective for monitoring thickness and composition. Currently, for example, optical techniques achieve precision of 0.4% RSD for thin (e.g., less than 20 angstroms) un-nitrided oxide (e.g., SiO2) films and 1.5-10% RSD for nitrided (e.g., SiON) oxide films.
TEM or STEM combined with electron energy loss spectroscopy (EELS) measurements can also provide thickness and some composition information. However, there are a number of issues that make the TEM impractical for use in production monitoring. For example, thickness measurement precision is typically greater than 2 angstroms and the cost of the needed equipment is generally prohibitive. Further, the length of time needed to perform such measurements is long (e.g., four hours per measurement) and a highly skilled specialist to prepare the sample and perform the measurements is typically a requirement.
Further, for example, SIMS, which has a very small sampling depth, is routinely used to quantify low level dopants and impurities in thin films (e.g., thin films less than 10 nanometers) because of this technique""s extreme surface sensitivity (e.g., single atom layer sensitivity and ppm-ppb detection limits). However, sensitivity factors used for SIMS quantification are matrix dependent and accurate quantification requires the use of calibrated reference samples. For example, when the concentration of a dopant exceeds 1%, it becomes a significant part of the matrix further complicating the task of quantification. To monitor silicon oxynitride gate films via SIMS, it would be required to regularly (e.g., at least daily) analyze reference silicon oxynitride films with thickness and nitrogen dose certified by an external direct measurement technique such as XPS.
Further, AES has also been used for thin film characterization. However, the high intensity electron beam used to make Auger measurements can alter the apparent composition of a thin film by causing chemical damage (e.g., can damage SiO2 films) or causing the migration of elements within the thin film. For example, there are concerns over the possible mobility of nitrogen within a film under the influence of an electron beam (e.g., nitrogen is known to migrate to the interface of an oxide-nitride stack (ONO) provided on silicon).
XPS, or ESCA, has been previously used to characterize thin films (e.g., ultra thin films less than 5 nanometers) such as lubricant coatings on computer hard disks with a measurement precision of 5% RSD. Further, characterization of other types of films such as SiON via XPS using standard practices has resulted in measurement precisions of 0.5% to 1.0%. For example, such standard practices involve the collection of data at relatively low analyzer angles such that depth resolution is enhanced. Such a low analyzer angle is typically less than 20 degrees. Use of a low analyzer angle generally results in a slow characterization process and also may result in problems associated with placement of the analyzer of the characterization system relative to the sample being analyzed. Yet further, existing data reduction methods which operate on the XPS collected data employ software tools for background subtraction and peak fitting that require frequent operator input making the results operator dependent and less precise.
In general, many of the techniques described above for characterizing thin films are invasive techniques, e.g., they involve destruction of at least one or more portions of the sample. Such techniques, e.g., those that use removal of material during depth profiling, are sufficient in many circumstances, e.g., research and development, product testing, etc., but do not provide for the ability to quickly analyze a thin film such as is necessary in production processes. For example, in such production processes, a thin film being formed typically needs to be analyzed so that such information can be used for production control, product test, etc., without loss of product due to invasive characterization of such films.
Nitrogen doped or nitrided silicon oxide is one material that is used as a gate oxide for a transistor structure. Such gate structures are only one of the growing number of semiconductor related material structures under development that require characterization at an unprecedented level of complexity. Such challenges are not limited to merely a desire for near-atomic and monolayer spatial resolution, but are magnified by the level of accuracy, precision and speed demanded by the semiconductor fabrication industry.
There is a distinct need to develop adequate characterization methods and systems. The ability to characterize materials at such levels is necessary to enable product development and also necessarily precedes evolution of process control. For example, there is a need for suitable systems and methods to provide parametric thickness, nitrogen dose, and nitrogen distribution information for thin nitrided silicon oxide films such as used for transistor gate oxides.
Systems and methods according to the present invention for characterizing samples are described herein. In particular, such systems and methods are particularly beneficial for the non-destructive analysis of thin films. As used herein, a thin film is generally defined as being less than about 10 nanometers in thickness. The present invention is also particularly beneficial for use with thin films having a thickness that is less than about 4 nanometers.
The present invention can provide, for example, an accurate measure of a component concentration (e.g., an elemental and/or a chemical species) in a thin film, thickness of such a thin film, distribution or uniformity of a component concentration across the thin film, and/or uniformity of thickness across such a thin film. Further, the present invention can detect process excursions based on the determination of thickness of a film and/or detect a process excursion by detecting a change in nitrogen concentration determined by measurement of nitrogen signal and correlation of that measurement with film thickness. Such change in nitrogen concentration may be due to either a change in total nitrogen in the film or a change in the nitrogen depth distribution.
The present invention may employ novel data reduction that includes, for example, a fitting process to compare measured peak shapes for elemental and/or chemical species (e.g., Si peak shapes previously measured for a particular process to be monitored) to collected XPS data, e.g., using a non-linear least squares fitting algorithm. Such fitting is performed without the necessary conventional operator input. In such a manner, e.g., with elimination of frequent operator input that conventionally made the results operator dependent and less precise, enhancement of the data quality yielding improved precision is accomplished.
A method for use in characterizing a film according to the present includes providing at least one measured spectral peak shape (i.e., measured basis spectra) representative of a concentration of at least one component of a film (e.g., a dielectric film such as a silicon oxynitride film and/or a film having a thickness of less than about 10 nanometers, and even less than about 4 nanometers), wherein the film is formed on a substrate by a particular process defined by a set of processing conditions. An acquired spectrum is provided for an additional film to be characterized, wherein the additional film is formed on a substrate by the particular process defined by the set of processing conditions. The at least one measured spectral peak shape is compared to the acquired spectrum and at least a thickness measurement is determined for the film to be characterized based on the comparison.
In one embodiment of the method, a spectral background is calculated for the acquired spectrum and the at least one measured spectral peak shape and the spectral background are compared to the acquired spectrum.
In yet another embodiment of the method, the at least one measured spectral peak shape includes a measured spectral peak shape representative of a concentration of the at least one component in the film and at least another measured spectral peak shape representative of a concentration of the at least one component in at least a portion of the substrate. Further, for example, the acquired spectrum may include overlapping peak areas representative of a concentration of the at least one component (e.g., silicon) in the additional film and the at least one component (e.g., silicon) in at least a portion of the substrate. In such a case, the comparison of the at least one measured spectral peak shape and the spectral background to the acquired spectrum may include fitting the acquired spectrum to the measured spectral peak shape representative of a concentration of the at least one component in the film and the at least another measured spectral peak shape representative of a concentration of the at least one component in at least a portion of the substrate using the spectral background calculated from the acquired spectrum to extract separate acquired spectrum peak areas from the overlapping peak areas. One of the separate acquired spectrum peak areas is representative of a concentration of the at least one component (e.g., silicon) in the additional film and the another separate acquired spectrum peak area is representative of the at least one component (e.g., silicon) in at least a portion of the substrate.
In another embodiment of the method, providing the at least one measured spectral peak shape may include providing a high resolution spectrum. After subtracting a spectral background from the high resolution spectrum, at least one narrow measured spectral peak shape representative of a concentration of at least one component of a film and at least another narrow measured spectral peak shape representative of a concentration of the at least one component in at least a portion of the substrate can be separated. A broadening function can then be applied to the at least one narrow measured spectral peak shape and the at least another narrow measured spectral peak shape to match a resolution of the acquired spectrum.
In another embodiment of the method, surface spectral measurements for use in the process are provided by irradiating the additional film with x-rays resulting in the escape of photoelectrons, detecting the escaping photoelectrons, and generating a signal representative of the detected photoelectrons. The surface spectral measurements are based on the generated signal. An analyzer may be provided that includes an input lens receptive of photoelectrons with the input lens having a central axis extending therethrough. The input lens is positioned such that the central axis of the input lens is at an analyzer angle relative to a surface of a film, wherein the analyzer angle is in the range of about 45 degrees to about 90 degrees, preferably, in the range of about 60 degrees to about 90 degrees. Further, for example, irradiating the additional film with x-rays may include irradiating the additional film with x-rays from a low energy x-ray source less than 2000 eV.
A system for use in characterizing a film is also described according to the present invention. The system includes an x-ray source operable to irradiate one or more films with x-rays resulting in the escape of photoelectrons and an analyzer operable to detect escaping photoelectrons. The analyzer is operable to generate a signal representative of the detected photoelectrons for use in providing an acquired spectrum for one or more films. Further, the system includes a computing apparatus operable to recognize at least one measured spectral peak shape representative of a concentration of at least one component of a film, wherein the film is formed on a substrate by a particular process defined by a set of processing conditions. The computing apparatus is further operable to recognize an acquired spectrum for an additional film to be characterized, wherein the additional film is formed on a substrate by the particular process defined by the set of processing conditions. The at least one measured spectral peak shape can be compared to the acquired spectrum for use in determining at least a thickness measurement based thereon.
In other embodiments of the system, the system is operable to implement one or more of the functional processes described above and also elsewhere herein. Further, a program storage media, readable by a media read apparatus under control of a computer, tangibly embodying a program executable to perform a process for characterization of thin films is also described. The program is also operable for implementation of one or more of the functional processes described above and also elsewhere herein.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.